1) Field of the Invention
The field of the present invention relates to wireless communication systems and, more particularly, to techniques for synchronizing and selectively addressing multiple receivers in a wireless, spread-spectrum communication system.
2) Background
Spread spectrum communication, a technique for transmitting and receiving signal over a bandwidth wider than the data to be transmitted, has in recent years become widely for both military and commercial applications. Its advantages include, for example, resistance to interference, low power density (and hence minimal creation of interference) over the transmission frequency band, and security of communications.
The two most common spread spectrum communication techniques are generally referred to as direct sequence spread spectrum (DSSS) communication and frequency hopping spread spectrum (FHSS) communication.
Direct sequence spread spectrum communication involves direct sequence modulation of a carrier signal, which is a known technique for generating wide-band, low power density signals which have statistical properties similar to random noise. In a direct sequence spread spectrum communication system, the data to be transmitted is generally encoded in some fashion, in a manner which causes the signal to be “spread” over a broader frequency range and also typically causes the signal power density to decrease as the frequency bandwidth is spread. In a common method of direct-sequence spread spectrum modulation, a pseudo-random chip sequence (also called a pseudo-noise code sequence or a PN code sequence) is used to encode data which is then placed on a carrier waveform. The chipping rate of the pseudo-random sequence is usually much higher than the data rate. The resulting encoded signal is generally spread across a bandwidth exceeding the bandwidth necessary to transmit the data, hence the term “spread spectrum”.
At the receiver, the signal is decoded, which causes it to be “despread” and allows the original data to be recovered. The receiver produces a correlated signal in response to the received spread spectrum signal when it is able to match the chip sequence to a sufficient degree. To do so, the receiver generates the same pseudo-random chip sequence locally, synchronizes its chip sequence to the received chip sequence, and tracks the signal by maintaining synchronization during transmission and reception of data.
Frequency hopping spread spectrum communication also involves a pseudo-random (i.e., spreading) code, but the code is used to select a series of frequencies rather than as information for directly modulating a carrier, as is generally done in direct sequence spread spectrum communication. In a very broad aspect, frequency hopping spread spectrum communication may be viewed as a type of frequency shift keying, but with many more frequency choices which are selected by use of the spreading code. In what is known as “fast” frequency hopping, a number of frequency changes or “hops” are carried out during the time period of sending one or more data symbols—e.g., a set of data bits—wherein the number of frequency changes or hops is greater than the number of data symbols to be transmitted. In “slow” frequency hopping, on the other hand, one or more data symbols is transmitted during each hopping interval.
In one type of frequency hopping spread spectrum communication, the frequency hopping transmitter includes a code generator and a rapid-response frequency synthesizer capable of responding to the coded output from the code generator. During each frequency hopping interval, a set of selected code bits are used to determine which frequency will be transmitted. Data may be transmitted in any way available to other communication systems, and in either analog or digital form. For example, a number of discrete data bits may be transmitted during each frequency hopping interval. Alternatively, a single data bit may be transmitted over a large number of frequency hopping intervals.
A frequency hopping receiver, like the transmitter, also typically includes a code generator and rapid-response frequency synthesizer. The received frequency hopping signal is then mixed with a locally generated replica of the transmitted signal (which may be offset by a fixed intermediate frequency) such that modulation of the received signal and the locally generated replica produces a constant difference frequency when the transmitter and receiver are in synchronism. Once the spread spectrum modulation is removed, the de-hopped signal is then processed to demodulate the transmitted information.
Both direct sequence and frequency hopping spread spectrum communication techniques may be used in the formation of a multiple access communication system. Distinct spreading codes can be used to distinguish transmissions, thereby allowing multiple simultaneous communication. Different users within a wireless communication system may, using distinct spreading codes, thereby transmit simultaneously over the same frequency without necessarily interfering with one another, particularly if the codes in use are selected to be orthogonal with respect to one another. A multiple-access communication system in which transmissions are distinguished according to the code used to encode the transmission is sometimes referred to as a code division multiple access (CDMA) communication system, which may be either a direct sequence or a frequency hopping spread spectrum system.
In either a frequency hopping or direct sequence spread spectrum communication system, the requirement of synchronization by the receiver has generally been a problem in the art. This requirement generally increases the difficulty of initially acquiring a spread spectrum signal, especially in a noisy environment, and also can cause difficulty in tracking and/or maintaining spread spectrum communication after established. Synchronization and tracking requirements often translate into additional circuit complexity at the receiver and increased cost, and may impose operational constraints on the communication system. For example, the extra time required to achieve synchronization can degrade the efficiency of the communication system, and may be detrimental in systems requiring very rapid establishment of a communication link.
In a frequency hopping system, in the absence of synchronization, the receiver must monitor all possible frequencies due to the otherwise unpredictable nature of the frequency hopped signal, which forces the receiver to employ a large number of synthesizers or even an array of distinct receivers. For example, the receiver may need to sample all of the possible frequency inputs at once, and then to select the channel with the largest signal in a given frequency hopping interval as the correct one. In order to monitor each possible frequency, the received signal is envelope detected and then band-pass filtered at each of the possible frequencies, with the largest of the filtered signals during a frequency hopping interval being deemed the transmitted frequency at that instant. This type of receiver design, however, has the drawback of requiring a potentially large number of band pass filters, one filter for each possible frequency.
If proper synchronization is achieved, on the other hand, it is possible to use a single (typically relatively high speed) frequency synthesizer to demodulate the incoming frequency hopped signal.
One technique for attempting to acquire synchronization of a frequency hopped signal is disclosed in U.S. Pat. No. 6,148,020. According to the technique disclosed therein, a frequency hopping receiver repeatedly mixes a partial code string which is part of the spreading code sequence for frequency hopping with the received signal. The receiver then attempts to synchronize by judging the detection level of a predetermined frequency. When the partial code string to be mixed coincides with part of the original code sequence, the detection level is presumed to become sufficiently large such that the receiver is judged to be in synchronization with the transmitter.
An alternative to the above-referenced technique is to use a preamble to attempt to synchronize a frequency hopping receiver. Such a technique is disclosed, for example, in U.S. Pat. No. 6,084,905. As described therein, a frequency hopping transmitter transmits a continuous wave in a first field of a preamble field of a communication frame, then transmits a carrier which is modulated with a symbol timing signal in a second field in the preamble field, followed by transmission information. A timing recovery circuit in the receiver uses the preamble to help establish synchronization. In the particular system described in the foregoing patent, a synchronous frame is broadcast as a reference for each frequency hopping equipment in the communication system.
Besides difficulties in achieving synchronization in spread spectrum systems, it can also be challenging, particularly in multiple access communication systems, for a receiver to be aware of when a transmitter is attempting to transmit information to it. This can be particularly difficult in military and other applications in which secrecy is paramount.
In various situations it can be advantageous for a transmitter to be able to selectively transmit a broadcast transmission to only one or a few receivers from many possible receivers, and to exclude from reception those receivers to which the communication is not directed. However, few, if any, techniques exist for such selective broadcast. No known technique exists for selectively broadcasting a transmission only to one or a specified group of receivers from many possible receivers, without at least requiring participation of the receivers in the excluded group.
It would therefore be advantageous to provide a communication technique that provides improved synchronization and the capability of selectively addressing receivers, and which overcomes the drawbacks, disadvantages or limitations of conventional techniques.